This invention relates to an improvement in a gas turbine, and more particularly to an improved gas turbine of the type in which part of compressed air for combustion is used as cooling air for cooling moving blades.
In a conventional gas turbine of this general type as disclosed in Japanese Patent Unexamined Publications Nos. 57-26206, 1-83827 and 48-87212 and Japanese Utility Model Unexamined Publication No. 64-8504 usually, part of compressed air for combustion is withdrawn, and moving blades are cooled by the withdrawn air. This construction will now be described more specifically with reference to FIGS. 4 and 5. The gas turbine broadly comprises a combustor 1A disposed on the stationary side, a compressor 2A for producing compressed air which is used in the combustor 1A, and a turbine 3A driven by combustion gas from the combustor 1A.
The compressor 2A is driven by the turbine 3A to produce high-pressure air which is supplied as combustion air to the combustor 1A. The combustor 1A causes a chemical reaction between this air and fuel, that is, combusts the fuel. Hot gas (1,100.degree. to 1,400.degree. C.) resulting from the combustion flows into the turbine 3A to perform the work. The work is given in the form of rotary energy, and, for example, it is converted into electric energy if the load is a generator.
First-stage moving blades 10a and second-stage moving blades 10b of the turbine 3A which blades receive the hot combustion gas must be used within the high-temperature strength limits of the material of these moving blades. For this reason, these moving blades are usually cooled by air a flowing through an internal bore in a shaft.
Japanese Patent Unexamined Publication No. 60-101202 discloses one example of such cooling method, and more specifically describes return flow-type cooling moving blades, each of which is hollowed so as to have a cavity therein and has therein a turbulence promoter for promoting heat transfer.
In the gas turbine, the air a of high pressure emerging from the outlet of the compressor must be used as cooling air for cooling the first- and second-stage moving blades 10a and 10b of the turbine.
More specifically, this cooling air is finally discharged from the first-stage moving blades into the combustion gas of high pressure via cooling holes in the moving blades. Therefore, it is necessary to use the cooling air of high pressure, emerging from the outlet of the compressor, in order to prevent a reverse flow (i.e., flow from the exterior of the moving blade toward the interior thereof).
The cooling of the moving blades is thus effected by discharging part of the high-pressure air, outputted from the compressor, through many cooling holes formed on the surface of each moving blade thereby resulting in a so-called "shower cooling". In this case, because of such many cooling holes on the surface of the moving blade rotating at high speed, a surface loss is increased, and besides since the high-pressure gas is injected from the cooling holes, this loss is still further increased. More specifically, the combustion gas, when passing past the moving blades, is influenced by the cooling air injected from the cooling holes on the moving blade, that is, the combustion gas is subjected to disturbance resulting in the moving blades suffering a so-called blade profile loss.
If the moving blades can be sufficiently cooled by the cooling gas of a very low temperature, the number of the cooling holes formed on the moving blades can be reduced, and the loss due to these cooling holes is reduced. However, the cooling air for the moving blades must have a high pressure in order to prevent the above-mentioned reverse flow, and the higher the pressure of the cooling air, the higher the temperature thereof. Therefore, in order to sufficiently cool the moving blades, a large amount of cooling air must flow past the moving blades, and it is very difficult to reduce the number of the cooling holes on the surface of the moving blades.